Conventional mobile phones provide manual adjustment of the loud speaker sound level by means of a press button or a sliding button etc.
Assuming a first case where a mobile phone detects an incoming call, and a ring signal is generated in the phone. For answering the call, the user has to press a key on the keypad of the phone in order to generate an off-hook signal. Then, he puts the phone to his ear and starts to speak with and listen to the calling party.
In a second case, when the user does not want to or can put the telephone to his ear immediately on answering the call, he similarly makes an off-hook signal, but answers the call without immediately moving the phone to his ear. Analog sound wave signals are received in the microphone and they are converted to digital signals before they are sent to the calling party. If the loudspeaker volume is set to providing a normal level of the sound (suitable when the phone is held close to the ear), the user will not be able to hear the calling party. Hence, the user has to increase the volume by manually adjusting the volume by means of the press button or sliding button. However, when the user has increased the volume to a level appropriate for a distance of about 50 cm, between the ear and the loudspeaker, and then moves the phone close to his ear, the volume will be too high. Hence, the user will have to decrease the sound level manually.
In these cases it is desirable to have a mobile phone which automatically detects when the phone is close to the ear or when it is at some distance from the ear and which adjusts the sound level accordingly. This function is, however, not provided by conventional prior art mobile phones.
JP-9/252333 discloses an audio conference device providing a constant listening level even if the position of a voice input device is changed in relation to a voice output device. This object is solved by installing an ultrasonic wave oscillating part in the vicinity of the voice output device and installing an ultrasonic wave receiving part in the vicinity of the voice input device and comparing a received signal with a reference signal. The distance between the microphone and the speaker is calculated from a pulse time difference between a pulse signal transmitted from the output device to the input device and a corresponding reference signal. Then, the speaker sound volume is regulated in accordance with the distance.
This solution is, however, not applicable in a mobile phone application because the microphone and the loudspeaker is mounted in a casing in the mobile phone and the distance between the microphone and the loudspeaker is fixed and does not change.
U.S. Pat. No. 4,490,584 discloses a telephone system having a remote microphone and an associated transmitter, and a network located receiver for signals outgoing over the telephone network and including a local loudspeaker to broadcast signals incoming over the telephone network, wherein the loudspeaker audible level is controlled to vary with the level of the received microphone signal. The level of the loudspeaker signal is increased when the received microphone signal increases, and vice versa, allowing the user to control the loudspeaker level by adjusting mouth-to-microphone distance or speech loudness. This solution to control the loudspeaker level is, however, not applicable or useful in a mobile phone in order to provide an automatic adjustment of the sound level in the loudspeaker. A high speech loudness or short mouth-to-microphone distance increases the loudspeaker audible level, which can impair the hearing of a person using such a phone. Another reason for not using this proposed solution is that the loudspeaker level only responds to the received microphone signal.
GB-A-2 203 315 discloses a multi-phonic balancer having measuring devices mounted on each speaker arranged to determine the relative distance of the speakers from a listener by means of signals reflected from the listener. Further, control means are arranged to vary the respective volumes of sound reproduction from the system. However, the distance measurement is limited to either ultrasonic or infra red techniques.